This invention relates to a method for controlling the release of the radioiodine produced during the process for the production of molybdenum 99 by fission of uranium 235 (U.sup.235).
Molybdenum 99 (Mo.sup.99) is useful as an isotope generator of six-hour technetium 99 m which is a widely used radioisotope in diagnostic medicine. Molybdenum 99, having a high specific activity and being radio chemically pure, is conveniently prepared from the fission of U.sup.235. The U.sup.235 fission is brought about by neutron irradiation producing among the isotopes those of molybdenum and iodine. Other fission by-products, such as zirconium, niobium and ruthenium can also be present. The specific activity of the fission products depends upon the irradiation time, neutron flux of irradiation and the decay period following reactive discharge.
Briefly, molybdenum 99 is produced by irradiating an aluminum-uranium alloy in a neutron flux of about 2.times.10.sup.14 neutrons per square centimeter per second (n/cm.sup.2 sec). Generally, following irradiation for an extended period of time, the aluminum-uranium alloyed target is allowed to decay for a short period of time, such as eight hours following reactor discharge, to reduce the amount of short-lived fission products.
The target is reacted in strong caustic, such as sodium hydroxide to release fission-product gases. The resultant solution is filtered to remove uranium and insoluble fission by-product hydroxides. Following filtration, the filtrate is acidified with a strong acid such as sulfuric acid. The acid is added to volatilize any radioiodine produced during the fission process. Various radioiodine isotopes are produced during the fission of U.sup.235. The longer half-life isotopes formed are I.sup.131, I.sup.132, I.sup.133 and I.sup.135. The acidic residue is extracted with bis(2-ethylhexyl)phosphoric acid to extract Mo.sup.99. The ensuing organic and aqueous phases are separated and the organic phase is washed with hydrochloric acid. The hydrochloric acid wash is used to remove aluminum, rare earths and alkaline earth fission products that can be present in small amounts as well as entrained aqueous feed solution. In addition, iodine 131-135, zirconium 95, niobium 95 and ruthenium 103-106 are partially extracted from the organic phase by the hydrochloric acid solution.
The free radioiodine liberated from the process solution by acidification is volatilized and collected in a scrubbing process wherein the scrubbing solution is a solution of sodium hydroxide and sodium sulfite. As iodine is highly volatile, charcoal filters are generally positioned along the flow path of the process to absorb any radioiodine not removed by the scrubbing process.
The procedure practiced in the state-of-the-art process for removing radioiodine presents problems due to the volatile nature of the radioiodine. Special precautions must be taken to prevent the release of the volatile radioiodine into the atmosphere. Care must also be taken in the handling of the waste material from the scrubber process as such waste material contains radioactive material. This scrubber waste material also presents a handling and disposal problem in that it is a relatively large volume in relation to the volume of the caustic solution used to react with the aluminum-uranium alloy target.